A multi-wire saw with which a large number of wafers can be cut out at once with a relatively small cutting allowance has been used for cutting a silicon ingot. The multi-wire saw is configured such that a wire fed from a wire-feeding mechanism is wound around between two wire guide rollers at a pitch of about 0.3 to 0.4 millimeter, and the wire is wound up by a wire winding-up mechanism. A tension roller that constantly applies a predetermined tension to the wire is provided in between the wire-feeding mechanism and the wire guide rollers, and in between the wire guide rollers and the wire winding-up mechanism. In the thus configured multi-wire saw, the wire-feeding mechanism, the two wire guide rollers and the wire winding-up mechanism are synchronously controlled to be driven and the position of the tension roller is controlled so that the wire is fed at a speed of about 600 m/min, and slurry is applied to the wire. In this state, an ingot that is fixedly adhered to a working plate is fed downward. Then, abrasives are pressed against the ingot with the wire and the ingot is rolled so that micro-cracks are produced on a surface layer of the ingot and scraped to be removed as silicon fine particle dust. Thus, the silicon ingot is cut. When cutting the silicon ingot, the cutting allowance is required to be reduced and wafers is required to be thinner in order to achieve a higher yield of wafers and cost reduction of a material for wafers.
In order to reduce the cutting allowance, a wire diameter can be made smaller. However, because the breaking strength of the wire is proportionally decreased in this case, tension applied to the wire needs to be reduced. The reduced tension weakens force that allows the wire to be wound around the wire guide rollers, and the wire is displaced in guide grooves that are formed on the wire guide rollers for receiving the wire. Thus, it results in varying in thicknesses of wafers cut out from the ingot.
In manufacturing a solar cell, a positioning member is pressed against an edge of a wafer to position the wafer and to perform a predetermined process, and then the wafer is transferred to the subsequent process step. A positioning member is pressed against the edge of the wafer again to position the wafer. Such processes are repeatedly performed for a large number of times. Therefore, a wafer having a thin edge cannot withstand such pressing force that is repeatedly applied thereto, resulting in producing cracks or chips on the edge of the wafer and the wafer becomes defective.
In order to cut out sinner wafers, a pitch of the wound wire needs to be narrowed. However, the narrower pitch makes the guide grooves formed on the wire guide rollers shallow, so that increasing the probability of disengagement of the wire from the guide grooves increases. The disengagement of the wire from the guide grooves causes wafers near the wire not to be cut out. In the worst scenario, the wire might be tangled and broken and thus non-defective wafers cannot be obtained at all.
These problems, such as the displacement in the guide grooves and the disengagement of the wire from the guide grooves on the wire guide rollers, predominantly arise after completion of cutting the silicon ingot. For example, when the ingot is disengaged from the wire (the wire is withdrawn from the ingot) after the completion of cutting the silicon ingot, the wire is caught and drawn by the ingot to be disengaged by being lifted from the guide grooves on the wire guide rollers. In order to prevent the wire from being displaced in and disengaged from the guide grooves, a restraining unit that holds the wire near the ingot access area to the wire after the completion of cutting of the silicon ingot is proposed (see Patent Document 1). This restraining unit is configured as a pair, and moves according to variation of the outer diameter or movement of the ingot so as to be arranged near the access area to the wire at any time when the wire is withdrawn. Further, the unit serves as a slurry supplying unit.
[Patent Document 1] Japanese Patent No. 3083232